style: Apply black v26 formatting

[amimas]

The CI environment uses black v26, which reported formatting errors against the codebase. This is why currently PRs are not mergeable.

This commit applies the updated formatting rules to align with the CI and resolve the linting failures.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.18%. Comparing base (9a75e56) to head (8027b0a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
gitlabform/configuration/core.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1176      +/-   ##
==========================================
+ Coverage   86.32%   87.18%   +0.85%     
==========================================
  Files          73       73              
  Lines        3635     3635              
==========================================
+ Hits         3138     3169      +31     
+ Misses        497      466      -31     

Flag	Coverage Δ
integration	85.58% <66.66%> (+1.10%)	⬆️
unittests	37.19% <33.33%> (ø)

Files with missing lines	Coverage Δ
gitlabform/__init__.py	71.58% <ø> (ø)
gitlabform/configuration/__init__.py	100.00% <ø> (ø)
gitlabform/configuration/transform.py	96.49% <ø> (ø)
gitlabform/lists/filter.py	97.72% <ø> (ø)
gitlabform/processors/abstract_processor.py	87.87% <100.00%> (ø)
gitlabform/configuration/core.py	81.98% <50.00%> (ø)

... and 4 files with indirect coverage changes

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